The initial contraction of skinned muscle fibers on photorelease of ATP in the presence of ADP.

: 1) We photoreleased ATP in skinned muscle fibers from rat psoas, and examined the effects of ADP on the ATP-induced isometric transient. 2) On release of ATP, the muscle stiffness initially falls at any [Ca2+]. ADP slowed this stiffness decrease, suggesting competition between ADP and ATP. Even in the presence of ADP, the rate of stiffness decrease was proportional to the amount of released ATP, but was not affected by Ca2+, P(i), or the fiber tension. 3) In the absence of ADP, the Ca2+ sensitive contraction occurs only after the initial phase. However, in the presence of ADP, the fiber developed tension during the initial phase before the late contraction. The initial contraction was insensitive to Ca2+, resistant to P(i) and BDM, larger on releasing more ATP, but not markedly influenced by the fiber tension. 4) Our results seem to suggest that, in the physiological reaction cycle, the cross-bridges develop significant force immediately after the binding of ATP, before releasing P(i).